1. Field of the Invention
This invention relates to controlling video devices.
2. Description of the Prior Art
It has been proposed that multiple video devices--for example, video tape recorders (VTRs), switchers and so on--be controlled by computer. One way in which this can be done is for the computer to issue so-called RS-422 commands to the video devices using the established RS-422 control interfaces and protocols.
However, in a complex system, it may be necessary to issue RS-422 commands simultaneously to several devices. As well as the requirement for issuing the commands simultaneously, a more onerous requirement is that the commands often have to be timed to an accuracy of a fraction of a field or frame period with respect to an external source of video syncs (synchronisation pulses).
Present day computers are often standard PC-compatible general purpose computers, operating under a multitasking operating system such as the Microsoft Windows NT operating system or the so-called QNX operating system. These operating systems allow message-driven multitasking between multiple concurrent processing tasks, which provides efficient use of the computer's processing capacity. However, this does mean that the time at which a particular operation takes place is not generally predictable to the accuracy of a fraction of a video field period required for issuing control RS-422 commands to the controlled video devices.
Thus, there are two apparently conflicting requirements: one is that the control commands have to be issued at very precise times, and the other is that an efficient multitasking arrangement leads to an uncertainty over the exact time at which a processing operation (such as the issue of an RS-422 command) will be performed by the computer.